Planet or not planet

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Planet or not planet
Planet or not planet

Planet or not planet - that's hardly the question anymore

In the last three years, reports of discoveries of planets outside our solar system have increased. Few of the observations withstood critical scrutiny. Solid candidates included 51 Pegasi B until 1997, when properties of the star itself were blamed for the observed signals. But the doubters of that time are now withdrawing their reservations. The search for planets orbiting stars other than our sun takes a dramatic turn this week as a key discovery, once caught in the crossfire of skeptics, has been redeemed. The contested candidate orbits the Sun-like star 51 Pegasi, (Image, 46k) which is 15 parsecs (49 light-years) from us. His name is simply 51 Pegasi B.

Its characteristics, however, are anything but ordinary: discovered by Michel Mayor and Didier Queloz of the Geneva Observatory in 1995, the celestial body has a mass half that of Jupiter and yet orbits 51 Pegasi in just 4, 2 days. This means that the planet's average distance from its star is only 7.5 million kilometers. For comparison: Mercury, the planet closest to the sun in our system, separates a total of 58 million kilometers from the sun.

Since this discovery, which was sensational at the time, four more so-called "epistellar planets of Jupiter" have been discovered, each of which belongs to a different star. Various other putative planets with more ordinary orbits (compared to our solar system) have also been discovered.

Skepticism initially focused on technical aspects of the method used to discover 51 Pegasi B. At present it is not possible to point a telescope into the sky and discover a planet. The problem is that stars are billions of times brighter than their planets, and when viewed from many light-years away, the star and planet are very close together. It's like trying to spot a firefly sitting next to a headlight from miles away.

Therefore, researchers must use more subtle means. To do this, they chose a technique in which they look for systematic deviations in the positions of characteristic lines in the spectra of the stars. These lines are ultimately the result of the excitation of chemical elements in the atmospheres of the stars. The wavelengths at which the lines appear reveal very precisely which element it is. Systematic shifts in the wavelengths of certain lines are caused by the Doppler effect as the star wobbles around the gravitational center of the stellar system as a whole. The unsteady migration is thus caused by invisible masses in the system, presumably by planets. Details of how the spectral lines move back and forth over days and months allow scientists to ascertain the existence of the planet or planets, as well as determine their masses and orbital parameters. Large planets that orbit very closely to the star - planets such as 51 Pegasi B - produce the clearest signals.

But bizarre propositions like this require particularly close scrutiny, David F. Gray of the University of Western Ontario pointed out as he suddenly entered the scientific arena. In a 1997 article in Nature, he argued that the lines in the spectra of 51 pegasi did not change position, but broadened and narrowed again in a cycle of 4.23 days, so this observation cannot be corroborated fits into a planetary orbit. Such a phenomenon, in his view, cannot have been caused by a planet orbiting the star, but rather by some kind of periodic change in the star itself was discovered using the technique of Doppler shift - and that was practically all of them.

One recognizes a true scientist by the fact that he also admits possible errors. Gray fully agrees, presenting new data in a report in the most recent issue of Nature (January 8, 1998) where this deformation of the lines does not occur. Gray admits the possibility that his original data show noise. However, he considers the likelihood of this to be low. With the new evaluation, he has regained the upper hand. Another skeptic, Artie P. Hatzes of the University of Texas, Austin, and his colleagues confirmed the recent findings in a separate paper in the same issue of Nature. Taken together, then, the research suggests that the presence of a planet best explains the spectral data after all.

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